It is important that children’s speech in stories sounds authentic. Children will soon notice if it is not. The Resources page on this website contains a selection of words that children commonly use. They should not be used in a haphazard way. It is a good idea to ‘allocate’ certain words which will be used uniquely by a character. For example, ‘epic’ and ‘random’ in the Charlie Lupton books are used only by Mickey.
A distinction must be made between how children speak and the narrator’s voice. I would not use these words as the third person narrator. For example, I would not say, ‘The school trip was the best evs.’ In The Osprey Enigma the narrator, Beverley, is writing in the first person, but she does not use ‘children’ words as the narrator because, although she is writing of when she was younger, she is writing as an adult.
I don’t think we should encourage children to write in this colloquial way. The narrator voice should set the right example. And it is also possible for adult characters to be used to comment on children’s speech when there may be scope for more formality. For example, in Charlie Lupton and the Orchid Princess, when Mickey has said ‘yep’ in class in reply to a question, Mrs Drake, the teacher says, ‘The word is yes Mickey, not yep’ (p. 11). But this, of course, should only be done sparingly. Otherwise, basically, it would be like, totally boring.